For the vast majority of DSLRs and mirrorless cameras, the base ISO is 100, although a few high-end Nikon cameras have a base ISO of 64.

The base ISO isn’t necessarily the lowest ISO setting. For example, my Canon 5D III has an ISO 50 setting, but this is achieved by reducing the gain on the sensor.

Since you get the highest quality images at the base ISO, it should be your default for any situation you can use it. If you can get the shutter speed you want and the aperture you want with ISO 100 (or ISO 64, check your camera’s manual to be sure), then that’s what you should use.

Note: The image above was shot on a Canon 650D at ISO 100. The sample images for each ISO value below are cropped versions of the same image shot at the stated ISO value.